Sheet-metal forming Subjects of interest doc

Chapter Sheet-metal forming Subjects of interest • Introduction/objectives • Deformation geometry • Forming equipments • Shearing and blanking • Bending • Stretch forming • Deep drawing • Forming limit criteria • Defects in formed parts Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Objectives • Methods of sheet metal processes such as stretching, shearing, blanking, bending, deep drawing, redrawing are introduced • Variables in sheet forming process will be discussed together with formability and test methods • Defects occurring during the forming process will be emphasised The solutions to such defect problems will also be given Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Introduction • Sheet metal forming is a process that materials undergo permanent deformation by cold forming to produce a variety of complex three dimensional shapes • The process is carried out in the plane of sheet by tensile forces with high ratio of surface area to thickness •Friction conditions at the tool-metal interface are very important and controlled by press conditions, lubrication, tool material and surface condition, and strip surface condition • High rate of production and formability is determined by its mechanical properties Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Classification of sheet metal parts (based on contour) 1) Singly curved parts (a) Singly curve (b) Stretch flange 2) Contoured flanged parts, i.e., parts with stretch flanges and shrink flanges 3) Curved sections (c) Shrink flange (d) Curved section 4) Deep-recessed parts, i.e., cups and boxes with either vertical or sloping walls 5) Shallow-recessed parts, i.e., dishshaped, beaded, embossed and corrugated parts (e) Deep drawn cup (f) Beaded section Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Classification of sheet metal forming (based on operations) Blanking Stamping Coining Folding Deep drawing Stretching Bending Suranaree University of Technology Roll forming of sheet Tapany Udomphol Ironing Wiping down a flange Jan-Mar 2007 Stress state in deformation processes • The geometry of the workpiece can be essentially three dimensional (i.e., rod or bar stock) or two dimensional (i.e., thin sheets) • The state of stress is described by three principal stresses, which act along axes perpendicular to principal planes • The principal stresses are by convention called σ1, σ2 and σ3 where σ1> σ2 > σ3 σ3 σ1 σ2 Principal stresses on an element in a three-dimensional stress state • Hydrostatic stress state is when σ1 = σ2 = σ3 Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 a) Uniaxial • Shear stresses provide driving force for plastic deformation b) Biaxial • Hydrostatic stresses cannot contribute to shape change but involve in failure processes c) Hydrostatic • Tensile crack growth or void formation • Compressive hinder crack, close void d) Triaxial Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Stress system in (a) sheet processes and (b) bulk processes • In sheet deformation processes (i.e., sheet metal forming, vacuum forming, blow moulding), the workpiece is subjected to two dimensional biaxial stresses (also depending on geometry) Suranaree University of Technology • In bulk deformation processes (i.e forging, rolling and extrusion), the workpiece is subjected to triaxial stresses, which are normally compressive Tapany Udomphol Jan-Mar 2007 Deformation geometry Plane stress • Principal stresses σ1 and σ2 are set up together with their associated strain in the x-y plane • The sheet is free to contact (not constrained) in the σ3 (z) direction There is strain in this direction but no stress, thus σ3 = 0., resulting in biaxial stress system • Since the stress are effectively confined to one plane, this stress system is known as plane stress Plane stress condition Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Plane strain • Deformation (strain) often occurs in only two dimensions (parallel to σ1 and σ2) • σ3 is finite, preventing deformation (strain) in the z direction (constrained), which is known as plane strain Example: the extrusion of a thin sheet where material in the centre is constrained in the z direction Plane strain condition Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Punch force vs punch stroke Punch force = Fdeformation + Ffrictional + (Fironing) Fdeformation Ffrictional Fironing Suranaree University of Technology - varies with length of travel - mainly from hold down pressure - after the cup has reached the maximum thickness Tapany Udomphol Jan-Mar 2007 Drawability (deep drawing) Drawability is a ratio of the initial blank diameter (Do) to the diameter of the cup drawn from the blank ~ punch diameter (DP) Limiting draw ratio (LDR)  Do LDR ≈  D  p Where η   ≈ eη   max …Eq.4 is an efficiency term accounting for frictional losses Normally the average maximum reduction in deep drawing is ~ 50% Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Practical considerations affecting drawability • Die radius – should be about 10 x sheet thickness • Punch radius – a sharp radius leads to local thinning and tearing Clearance between punch and die should be about 2040% > sheet thickness • Hold-down pressure – about 2% of average σo and σu • Lubrication of die side - to reduce friction in drawing • Material properties - low yield stress, high work hardening rates, high values of strain ratio of width to thickness R • Since the forming load is carried by the side wall of the cup, failure therefore occurs at the thinnest part • In practice the materials always fails either at (a) the shoulder of the die and (b) the shoulder of the punch Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Practical considerations for round and rectangular shells • Different pressures (tension, compression, friction, bending) force the material into shape, perhaps with multiple successive operations www.drawform.com Round shell • Different flow patterns at sides and corners • Corners require similar flow as round shells while sides need simple bending • The corner radii control the maximum draw depth • Centre to center distance of corners ≥ x corner radius • Bottom radius ≥ corner radius Suranaree University of Technology Rectangular shell Tapany Udomphol Jan-Mar 2007 To improve drawability • To avoid failures in the thin parts (at the punch or flange), metal in that part need to be strengthened, or weaken the metal in other parts (to correct the weakest link) • If sufficient friction is generated between punch and workpiece, more of the forming load is carried by the thicker parts • Concerning about crystallographic texture (slip system), degree of anisotropy or strain ratio R The dependence of limiting draw ratio on R and work hardening rate, n Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 The plastic strain ratio R measures the normal anisotropy, which denotes high resistance to thinning in the thickness direction ln(w / ln(wow)/ w) R = ln(h / h) ln(ho / h) R= o o …Eq.5 Where wo and w are the initial and final width ho and h are the initial and final thickness But it is difficult to measure thickness on thin sheets, therefore we have ln(wo / w) R= ln( wL / wo Lo ) Suranaree University of Technology Tapany Udomphol …Eq.6 Jan-Mar 2007 Example: A tension test on a special deep-drawing steel showed a 30% elongation in length and a 16% decrease in width What limiting draw ratio would be expected for the steel? L − Lo = 0.30 Lo w − wo = −0.16 wo R= L = 1.30 Lo w = − 0.16 = 0.84 wo ln( wo / w) ln(1 / 0.84) ln 1.190 = = = 1.98 ln((w / wo )( L / Lo )) ln(0.84 × 1.30) ln 1.092 From Fig 20-16 Dieter page 673, the limiting draw ratio ~ 2.7 Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Forming limit criteria • Tensile test only provides ductility, work hardening, but it is in a uniaxial tension with frictionless, which cannot truly represent material behaviours obtained from unequal biaxial stretching occurring in sheet metal forming • Sheet metal formability tests are designed to measure the ductility of a materials under condition similar to those found in sheet metal forming Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Erichsen cupping test • Simple and easy • symmetrical and equal biaxial stretching • Allow effects of tool-workpiece interaction and lubrication on formability to be studied • The sheet metal specimen is hydraulically punched with a 20 mm diameter steel ball at a constant load of 1000 kg • The distance d is measured in millimetres and known as Erichsen number Results of cupping test on steel sheets Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 The forming limit diagram • The sheet is marked with a close packed array of circles using chemical etching or photo printing techniques Grid analysis (a) before (b) after deformation of sheet • The blank is then stretched over a punch, resulting in stretching of circles into ellipses Major strain ε1(%) 120 100 • The major and minor axes of an ellipse represent the two principal strain directions in the stamping Failure 80 ε2 • The percentage changes in these strains are compared in the diagram A 60 ε1 B ε2 AK steel Safe 40 ε1 20 • Comparison is done in a given thickness of the sheet -40 -20 20 40 60 Minor strain ε2(%) 80 100 Forming limit diagram Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Example: A grid of 2.5 mm circles is electroetched on a blank of sheet steel After forming into a complex shape the circle in the region of critical strain is distorted into and ellipse with major diameter 4.5 mm and minor diameter 2.0 mm How close is the part to failing in this critical region? Major strain ε1(%) 120 Major strain 100 4.5 − 2.5 e1 = × 100 = 80% 2.5 Failure 80 ε2 Minor strain A 60 ε1 ε2 Safe 40 B AK steel ε1 20 2.0 − 2.5 e2 = × 100 = −20% 2.5 -40 -20 20 40 60 Minor strain ε2(%) 80 100 Forming limit diagram The coordinates indicate that the part is in imminent danger of failure Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Defects in formed parts www.bgprecision.com Springback problem • Edge conditions for blanking • Local necking or thinning or buckling and wrinkling in regions of compressive stress • Springback tolerance problems Crack near punch region Suranaree University of Technology • Cracks near the punch region in deep drawing minimised by increasing punch radius, lowering punch load Tapany Udomphol Jan-Mar 2007 • Radial cracks in the flanges and edge of the cup due to not sufficient ductility to withstand large circumferential shrinking • Wrinkling of the flanges or the edges of the cup resulting from buckling of the sheet (due to circumferential compressive stresses) solved by using sufficient hold-down pressure to suppress the buckling • Surface blemishes due to large surface area EX: orange peeling especially in large grain sized metals because each grain tends to deform independently use finer grained metals • Mechanical fibering has little effect on formability • Crystallographic fibering or preferred orientation may have a large effect Ex: when bend line is parallel to the rolling direction, or earing in deep drawn cup due to anisotropic properties Earing in drawn can aluminium.matter.org.uk Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 • Stretcher strains or ‘worms’ (flamelike patterns of depressions) Associated with yield point elongation • The metal in the stretcher strains has been strained an amount = B, while the remaining received essentially zero strain • The elongation of the part is given by some intermediate strain A Stretcher strain in low-carbon steel A B Relation of stretcher strain to stress strain curve Suranaree University of Technology • The number of stretcher strains increase during deformation The strain will increase until the when the entire part is covered it has a strain equal to B Solution: give the steel sheet a small cold reduction (usually 0.5-2% reduction in thickness) Ex: temper-rolling, skin-rolling to eliminate yield point Tapany Udomphol Jan-Mar 2007 References • Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-100406-8 • Edwards, L and Endean, M., Manufacturing with materials, 1990, Butterworth Heinemann, ISBN 0-7506-2754-9 Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 ... a fluid supplied from one side of the draw blank A drawing of hydroforming setup with fluid supplied from to both sides of the materials • Used for sheet forming of aluminium alloys and reinforced... Determining factors are 1) volume of production 2) the complexity of the shape Suranaree University of Technology Tapany Udomphol Jan-Mar 2007 Rubber hydroforming • Using a pad of rubber or polyurethane... 2007 Forming method There are a great variety of sheet metal forming methods, mainly using shear and tensile forces in the operation • Progressive forming • Shearing and blanking • Rubber hydroforming